A binding machine has been known that is provided with a punching blade for punching a punched hole while forming a cut-and-raised piece in each of a plurality of sheets, a cutting blade provided adjacently to this punching blade and punching a cut hole for locking the distal end side of the cut-and-raised piece, a base portion for holding the punching blade and the cutting blade, and a punch stand disposed through a gap for sheet insertion on this base portion. By performing a punching operation so that the punching blade and the cutting blade held on the base portion penetrate the sheet inserted in the gap for sheet insertion and protrude to the punch stand side, a punched hole and a cut hole are formed in the sheet, and by performing a withdrawing operation so that the cutting blade and the punching blade are returned to the base portion side in a state where the distal end side of the cut-and-raised piece cut and raised from the punched hole to the punch stand side is engaged with the cutting blade, the cut-and-raised piece is penetrated into the cut hole so as to bind the sheets (See Patent Literature 1, for example).
However, in such binding machines, a cut-and-raised piece having a substantially constant width corresponding to a width dimension of the cut hole from the base end to the distal end and a semicircular distal end is engaged with the linear cut hole formed in the sheet. That is, the plurality of sheets bound by this binding machine are maintained in a joined state in which the cut-and-raised piece folds from the front surface side into the back surface side when the cut-and-raised piece penetrates the cut hole. Thus, if a force is applied in a direction to separate each of the bound sheets, the linear cut hole is largely opened in the vertical direction, and the state where the cut-and-raised piece is folded might be released relatively easily, resulting in the engagement between the cut-and-raised piece and the cut hole becoming disengaged, and the joined state is released. Particularly, on a sheet with small friction resistance, the above-described joined state can be easily released, and thus some measure is in demand.